In the field, it is often necessary to convert a position of an object relative to an origin into an electrical signal. This output signal, which can be analog or digital, is often then fed into other electrical devices. In process monitoring systems, for example, the signal can be input into an electrical indicator which outputs a display of the required data. Such data may include liquid pressure, linear position, tension, or any other like measurement. In process control systems, for example, the electrical signal can be used as feedback and input into the controller which responds, based on the nature of the signal, to control the position of the object relative to an origin.
Applications exist for inductive based measurements of linear displacement of a probe. In a device such as that disclosed by U.S. Pat. No. 4,667,158 to Redlich, the linear displacement of a metallic cylindrical core within a helical coil of wire causes corresponding changes in the amplitude of the voltage signal imposed across the coil. This voltage signal is then demodulated by a half-wave rectifier including a diode, a resistor and a low-pass filter. A reference signal is demodulated by a second identical half-wave rectifier. The demodulated reference signal and the demodulated coil voltage signal are fed into the inverting and non-inverting inputs of a differential amplifier having an output which drives an electrical indicating instrument.
As disclosed by the Redlich patent, a metal jacket may be positioned around the coil of a displacement transducer to confine magnetic flux generated by the coil, as a result of current being transmitted therethrough, and to shield the transducer from stray fields. Redlich indicates that, when using the shield, it is desirable to increase sensitivity by partially filling the space between the coil and the shield with bonded ferrite powder.
It has been determined through experimentation that use of a cylindrical shield, such as that disclosed by Redlich, that is substantially continuous throughout its length, tends to significantly degrade sensitivity. Thus, adequate sensitivity in measurement cannot be achieved with the Redlich arrangement unless the space between the coil and the shield is filled with bonded ferrite powder or the like. In practice, filling the space defined by the shield and the coil is disadvantageous, however, since it leads to increases in cost of materials and requires the use of additional steps during the process of manufacturing the transducer. Thus, there is a need for a shield arrangement in which acceptable levels of measurement sensitivity are maximized without significantly increasing manufacturing costs and procedure.
In a similar device, such as that shown in U.S. Pat. No. 3,991,393 to Becker, Jr., the movement of a shielded core within a coil alters the inductance of the coil. The shield is not moveable with respect to the core but is instead fixedly attached to the core. Means are provided for delivering a signal responsive to the position of the shield and the core relative to the electric coil.